Substrate of single crystal of oxide, superconductive device using said substrate and method of producing said superconductive device

ABSTRACT

According to the present invention, when a super-conductive thin film is formed on a substrate of a single crystal, a compound having a composition of SrNdGaO 4  and a K 2  NiF 4  type crystal structure is used as a material employable for the substrate. Alternatively, a single crystal composed of an oxide in which Ca, La and Cr are added to the foregoing compound is used as a material employable for the substrate. Then, a super-conductive thin film composed of an oxide is formed on the substrate by employing an epitaxial growing method. Thus, the present invention makes it possible to provide a super-conductive material having an excellent property of lattice alignment, a stable and high critical superconductivity temperature and a stable critical superconductivity electric current.

TECHNICAL FIELD

The present invention relates to a substrate of a single crystalcomposed of an oxide, a super-conductive device having the substrate ofa single crystal used therefor and a method of producing asuper-conductive device.

BACKGROUND ART

It has been heretofore said that a super-conductive phenomenon appearswith a most peculiar nature among various electromagnetical naturesexhibited by a certain material. In view of the foregoing nature, it isexpected that practical application of the super-conductive phenomenonwill largely be widened in near future by utilizing its natures such ascomplete electrical conductivity, complete antimagnetism, quantificationof a magnetic flux or the like.

A high speed switching element, a detecting element having highsensitivity and a magnetic flux measuring instrument having highsensitivity can typically be noted as electronic devices for which theaforementioned super-conductive phenomenon is utilized, and it isexpected that these devices are practically used in the wide range ofapplication.

For example, a thin film of Nb₃ Ge deposited on the surface of asubstrate by employing a plasma spattering method can be noted as atypical super-conductive material which has been hitherto used for aconventional super-conductive device. However, since the thin film ofNb₃ Ge has a critical superconductivity temperature of about 23° K., thesuper-conductive device can be used only at a temperature lower thanthat of a liquid helium. For this reason, when the liquid helium ispractically used for the super-conductive device, there arises asignificant problem that a cooling cost and a technical burden to beborn are increased because of a necessity for installing an equipmentand associated instruments for cooling and liquidizing a helium gas withthe result that practical use of the super-conductive device not only inthe industrial field but also in the household field is obstructed.Another problem is that an absolute quantity of helium source is smalland limited.

To obviate the foregoing problems, a variety of endevors have been madeto provide a super-conductive material having a higher criticalsuperconductivity temperature. Especially, in recent years, remarkableresearch works have been conducted for providing a super-conductive thinfilm composed of an oxide having a higher superconductivity temperature.As a result derived from the research works, a criticalsuperconductivity temperature is elevated to a level of 77° K. Thismakes it possible to practically operate a super-conductive devicehaving the foregoing super-conductive thin film used therefor whileusing an inexpensive liquid nitrogen.

To form such a super-conductive thin film composed of an oxide asmentioned above, a spattering method or a vacuum vaporizing/depositingmethod has been heretofore mainly employed such that thesuper-conductive thin film is deposited on the surface of a substrate ofa single crystal of MgO or SrTiO₃ which is preheated to an elevatedtemperature.

In addition, with respect to a single crystal employable for thesubstrate, attention has been paid to a sapphire, YSZ, a silicon, agallium arsenide, LiNbO₃, GGG, LaGaO₃, LaAlO₃ or the like.

However, it has been found that the conventional method of forming asuper-conductive thin film while having a substrate of a single crystalof MgO or a substrate of a single crystal of SrTiO₃ used therefor as asubstrate has problems that a critical superconductivity current (Jc)can not stably be elevated, and moreover a critical superconductivitytemperature (Tc) is kept unstable.

To form an epitaxial film having excellent properties, it is necessarythat a material employable as a substrate satisfactorily meets thefollowing requirements. (I) The lattice constants of substrate crystalsare close to that of thin film crystals. (II) A quality of the thin filmis not degraded due to mutual diffusion to and from the substrate duringan operation for growing an epitaxial thin film. (III) The materialemployable as a substrate has a melting temperature higher than atlowest 1000° C., since it is heated up to an elevated temperature. (IV)A single crystal having an excellent crystal quality is readilyavailable on the commercial basis. (V) The material employable as asubstrate has an excellent property of electrical insulation.

On the other hand, with respect to a super-conductive material composedof an oxide having a higher critical superconductivity temperature, manyoxides each in the form of a thin film such as a LnBa₂ Cu₃ O₇₋δ (δ=0 to1, Ln:Yb, Er, Y, Ho, Gd, Eu or Dy), a Bi-Sr-Ca-Cu-O base oxide, aTl-Ba-Ca-Cu-O based oxide or the like have been heretofore reported.

All the oxides as mentioned above have lattice constants a and b each ofwhich remains within the range of 3.76 to 3.92 angstroms. When acoordinate system for each oxide is turned by an angle of 45° so that itis visually observed in the turned state, √2a and √2b are recognized asa basic lattice, respectively. In this case, the lattice constants a andb are expressed such that they remain within the range of 5.32 to 5.54angstroms.

In contrast with the aforementioned oxides, a magnesium oxide (MgO)which has been widely used as a material for a substrate at present hasa lattice constant a of 4.203 angstroms and thereby a differentiallattice constant between the aforementioned oxides and the magnesiumoxide is enlarged to an extent of 7 to 11%. This makes it very difficultto obtain an epitaxially grown film having excellent properties. Inaddition, it has been found that things are same with a sapphire, YSZ, asilicon, a gallium arsenide, LiNbO₃ and GGG.

Further, in contrast with MgO, SrTiO₃ has a small differential latticeconstant relative to the super-conductive thin film composed of an oxidewherein the lattice constant remains within the range of 0.4 to 4%.Thus, SrTiO₃ is superior to MgO in respect of a lattice matching.However, SrTiO₃ is produced only with a Bernoulli method employedtherefor. In addition, SrTiO₃ has a very poor crystal quality, andmoreover it can be obtained only in the form of a large crystal havingan etch pit density higher than 10⁵ pieces/cm². This makes it difficultto form an epitaxial film having excellent properties on the surface ofa substrate having a poor crystal quality. It should be added that it ispractically impossible to obtain a substrate having large dimensions.

A single crystal of LaGaO has a lattice constant a of 5.496 angstromsand a lattice constant b of 5.5554 angstroms. Thus, it is expected thatthe single crystal of LaGaO₃ has an excellent lattice matching relativeto the super-conductive material composed of an oxide. However, sincephase transition takes place at a temperature of about 150° C., therearises a problem that a twin crystal is contained in the single crystal.For this reason, removal of the twin crystal becomes .a significantsubject to be solved when a substrate for forming a super-conductivethin film composed of a single crystal of LaGaO₃ is used practically.

In addition, since the single crystal of LaAlO₃ has lattice constants aand b of 3.788 angstroms, it is expected that it has an excellentlattice matching relative to the super-conductive material composed ofan oxide. With respect to the single crystal of LaAlO₃, however, it isvery difficult to practically form a single crystal because of a veryhigh melting temperature of 2100° C. Another problem is that a twincrystal is contained in the single crystal.

With respect to the conventional materials which have been heretoforeused as a substrate for forming a super-conductive thin film in theabove-described manner, there arise problems that each of theconventional materials does not have an excellent lattice matchingrelative to the super-conductive thin film, it does not satisfactorilymeet the requirement for easily purchasing a single crystal on thecommercial basis and thereby it is substantially impossible to produce astable super-conductive device.

The present invention has been made in consideration of the foregoingbackground. Therefore, an object of the present invention is to providea material employable for a substrate of a single crystal which makes itpossible to form an excellent epitaxial super-conductive thin film.Another object of the present invention is to provide a super-conductivethin film having a high quality which can be employed for asuper-conductive device without unstable superconductivity inherent to asuper-conductive thin film composed of an oxide.

DISCLOSURE OF THE INVENTION

According to a first aspect of the present invention, there is provideda substrate of a single crystal composed of astrontium-neodymium-gallium based oxide which is employable as amaterial for the substrate, wherein a composition of the substrate has aK₂ NiF₄ type crystal structure represented by the following formula.

Sr₁₋ x Nd₁₋ y Ga₁₋ z O₄₋ w

(-0.1<x<0.1, -0.1<y0.1, -0.1<Z<0.1,

-0.2<W<0.2)

Then, a super-conductive thin film composed of the foregoing oxide isformed on the substrate by employing an epitaxial growing method.

In addition, according to a second aspect of the present invention thereis provided a material employable for a substrate of a single crystal inwhich one or more selected from a group comprising a calcium having amol ratio relative to a strontium within the range of 0.00001 to 0.1mol, a lanthanum having a mol ratio relative to a neodymium within therange of 0.00001 to 0.1 mol and a chromium having a mol ratio relativeto a gallium within the range of 0.00001 to 0.1 mol are added to acompound of which composition is SrNdGaO₄ wherein the compound has a K₂NiF₄ type crystal structure.

Then, a super-conductive thin film composed of an oxide is formed on theforegoing substrate by employing an epitaxial growing method.

By the way, it was reported by Gordon et al that a sintered material ofSrNdGaO₄ had a K₂ NiF₄ type crystal structure and its lattice constant awas 3.82 angstroms (Joint Committee on Powder Diffraction Standards.(1972) NO 24-119).

As mentioned above, the hitherto reported super-conductive materialseach composed of an oxide having a high superconductivity temperaturehave lattice constants a and b each of which remains within the range of3.76 to 3.92 angstroms.

Therefore, the lattice constant of SrNdGaO₄ has a substantially middlevalue within the range of 3.76 to 3.92. When a coordinate system of thesuper-conductive oxide material of SrNdGaO₄ is turned by an angle of 45°so that it is visually observed in the turned state, √2a can berecognized as a basic lattice wherein a lattice constant is 5.40angstroms. However, in a case where √2a of the super-conductive thinfilm composed of an oxide is recognized as a basic lattice, the latticeconstant of the super-conductive thin film remains within the range of5.3 to 5.54 angstroms. Thus, a differential lattice constant relative tothe super-conductive thin film composed of an oxide of a single crystalof SrNdGaO₄ is very small, as represented by the range of -1.6 to 2.6%.In addition, since each of the hitherto reported super-conductivematerials is very similar in crystal structure to SrNdGaO₄ and it has avery excellent lattice matching relative to the super-conductive thinfilm composed of an oxide of a single crystal of SrNdGaO₄, it has allthe conditions as mentioned.

However, there has not been hitherto seen any report on a single crystalof SrNdGaO₄. For this reason, nobody knows anything as to whether thesingle crystal of SrNdGaO₄ can practically be produced or not.

In view of the foregoing background, the inventors conducted a varietyof research works for the purpose of producing a single crystal ofSrNdGaO₄ As a result derived from the research works, the inventorssucceeded in producing a single crystal of SrNdGaO₄ on the trial basisby employing a crucible cooling method.

Specifically, when the single crystal of SrNdGaO₄ was produced, amixture of pulverized materials which were mixed together such that amol ratio among SrCO₃, Nd₂ O₃ and Ca₂ O₃ was set to 2:1:1 was used as araw material. The mixture was calcined at 1200° C. and the calcinedmixture was crushed and ground to perform a press-molding operation withthe pulverized mixture. Thereafter, a green compact derived from thepress-molding operation was sintered at 1300° C. to obtain a sinteredcompact. The sintered compact was placed in a crucible made of aplatinum in which it was heated up to an elevated temperature higherthan a melting temperature of the raw material. Thereafter, the sinteredcompact was gradually cooled at a rate of 1° C./min.

As a result, a large plate-shaped single crystal having a substantiallysquare shape and a size within the range of 5 to 15 mm with its platesurface as a C plane could be obtained. Additionally, it was found thatthe raw material had a melting temperature of about 1480° C. This meansthat the resultant single crystal was a single crystal having asufficiently high melting temperature.

It was confirmed that the thus obtained single crystal had a compositionwhich remained within the range represented by the following formula.

Sr_(1-x) Nd_(1-y) Ga₁₋₂ O_(4"w)

(-0.1<x<0.1, -0.1<y<0.1, -0.1<z<0.1, -0.2<W<0.2)

Then, a super-conductive thin film composed of an oxide was depositedand grown in an epitaxial manner on the substrate of the single crystalof Sr_(1-x) Nd_(1-y) Ga_(1-z) O_(4-w) produced in the above-describedmanner by employing a spattering method, a vacuum vaporizing/depositingmethod or the like. As a result, a super-conductive thin film havingexcellent superconductivity could be obtained.

In addition, it was found that the single crystal of Sr_(1-x) Nd_(1-y)Ga_(1-z) O_(4-w) could be produced also by employing a Czochralskimethod.

For example, a (001) axis single crystal having a diameter of 25 mm anda length of 100 mm could be obtained by using a sintered material havinga composition of Sr_(1-2x) -Nd_(1+x) Ga_(1+x) O_(4+x) (0<x<0.1) as a rawmaterial. Specifically, the sintered material was heated and molten at ahigh temperature to produce a molten substrate. Then, the moltensubstrate was pulled up under an atmosphere containing an oxygen by 1%by volume under the operational conditions that a pulling-up speed wasset to 1 to 6 mm/hr and a rotational speed of crystal was set to 10 to60 rpm.

In addition to the Czochralski method, a zone melting method, aBridgeman method or the like could be also employed to produce the samesingle crystal as mentioned above.

Further, a super-conductive thin film composed of an oxide having veryexcellent properties could be produced by using a substrate of a singlecrystal of SrNdGaO₄ with impurities added thereto according to thesecond aspect of the present invention.

It is believed that this is attributable to the fact that the surface ofthe substrate was chemically activated by addition of the impurities andthereby an epitaxially grown film having excellent properties could beformed on the substrate.

Additionally, the inventors produced various kinds of single crystalseach composed of mixed crystals. As a result, the inventors found outimpurities which enabled only the surface state of the single crystal tobe kept good without any deterioration of a crystal quality as well aswithout any increase of difficulty associated with the technology forgrowing the single crystal. Additionally, the inventors found out alimit of addition of each of the impurities.

It was found that all the impurities, i.e, Ca, La and Cr had an effectfor varying lattice constants of the single crystal but the singlecrystal was not largely affected as long as a quantity of addition ofeach of the impurities remained within the aforementioned range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a super-conductive thin film inaccordance with a first embodiment of the present invention, and FIG. 2is a schematic view of a measuring apparatus in accordance with a secondembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described in detail hereinafter withreference to the accompanying drawings which illustrate severalpreferred embodiments of the present invention.

EMBODIMENT 1

First, description will be made below with respect to a method ofproducing a single crystal of SrNdGaO₄.

717.3 g of Nd₂ O₃ (having a purity of 99.99%), 541.2 g of SrCO₃ (havinga purity of 99.999%) and 400.8 g of Ga₂ O₃ (having a purity of 99.999%)were mixed together and the resultant mixture was calcined at 1200° C.so that it was subjected to decarbonizing. Thereafter, the mixture wascrushed and ground to perform a press-molding operation with thepulverized mixture.

A green compact derived from the press-molding operation was sintered at1300° C. under an environmental atmosphere, whereby a sintered productof Sr₀.9 Nd₁.05 Ga₁.05 O₄.05 having a weight of about 1500 g wasobtained.

The sintered product was placed in a crucible of an iridium having anouter diameter of about 80 mm, a height of about 80 mm and a wallthickness of about 2 mm so that it was molten by high frequency heating.It should be noted that a nitrogen atmosphere containing an oxygen of0.5 to 2% was employed for performing the high frequency heating. Thereason why the oxygen was added to the atmosphere consists insuppressing vaporization of a gallium oxide.

After the sintered product of Sr₀.9 Nd₁.05 Ga₁.05 O₄.05 was molten inthe above-described manner, a single crystal of SrNdGaO₄ was grown byemploying a Czochralski method for pulling up a molten substrate.

At first, a (100 ) axis single crystal of SrTiO₃ was used as a seedcrystal. After a single crystal of SrNdGaO₄ was obtained, a singlecrystal having a (001) orientation in the foregoing single crystal wasuses as a seed crystal. As a result, a (001) axis single crystal havinga diameter of 25 mm and a length of 100 mm could be obtained byperforming a pulling-up operation under the operational conditions thata pulling-up speed was set to 1 mm/hr and a rotational speed of thesingle crystal was set to 30 rpm.

The single crystal of SrNdGaO₄ which had been produced in theabove-described manner was sliced to successively produce wafers. Eachof the wafers was used as a substrate of a (001) plane single crystalhaving the wafer-shaped configuration. Then, a thin film of YBa₂ Cu₃O₇₋δ having a thickness of 1000 angstroms was deposited on thewafer-shaped substrate of a single crystal under an atmosphere of amixture of argon and oxygen (having a mixing ratio of 1: 1) by employinga RF magnetron spattering method. It should be noted that the singlecrystal of SrNdGaO₄ was used as a target material wherein a compositionratio of the single crystal was determined such that the compositionratio of the single crystal after formation of the thin film coincidedwith that of YBa₂ Cu₃ O₇₋δ. The operational conditions for the formationof the thin film were determined such that a gas pressure was set to 10Pa, an electric power was set to 300 W and a temperature of thesubstrate was set to 600° C.

After the thin film was deposited on the single crystal, the substratewas annealed for one hour at 900° C. under an atmosphere of oxygen.

Thereafter, as shown in FIG. 1, a thin film 2 of YBa₂ Cu₃ O₇₋δ,exclusive of the region which was transformed into a circuit portion 2sby employing an electronic beam radiating method or the like, was usedas a normally electrically conductive portion 2n while forming a patternfor the thin film of YBa₂ Cu₃ O₇₋δ to serve as a circuit portion 2s.Thus, a device such as a memory device constructed by usingsuper-conductive materials could be formed under the condition that thepattern 2s of the super-conductive thin film which had been formed onthe substrate 1 of the single crystal of SrNdGaO₄ in the above describedmanner was used as a basic structural component.

A zero resistance temperature T_(cO) of the thin film of YBa₂ Cu₃ O₇₋δformed in the above-described manner and a critical superconductivityelectric current J_(c) of the same at 77° K. were measured by employinga four terminal method.

The results derived from the measurements are shown in Table 1.

For the purpose of comparison, a thin film of YBa₂ Cu₃ O₇₀δ wasdeposited on the (100) plane single crystal of SrTiO₃ which had beenheretofore used as a substrate under the same operational conditions asmentioned above. In addition, the results derived from measurementsconducted for the zero resistance temperature T_(cO) and the criticalsuperconductivity electric current J_(c) at 77° K. at the time when theforegoing thin film was deposited in the above-described manner areshown in Table 1.

                  TABLE 1                                                         ______________________________________                                        material of substrate                                                                           T.sub.c                                                                              J.sub.c (A/cm.sup.2)                                 ______________________________________                                        (SrTiO.sub.3)     79     0.5 × 10.sup.4                                 SrNdGaO.sub.4     85       1 × 10.sup.5                                 ______________________________________                                    

As is apparent from the results shown in Table 1, the case where thesingle crystal of SrNdGaO₄ was used as a substrate is superior inrespect of T_(cO) and J_(c) to the case where the single crystal ofSrTiO₃ was used as a substrate. It is considered that the reason why theformer case is superior to the latter case consists in that thesubstrate of the single crystal of SrNdGaO₄ had an excellent property ofcrystallization of the thin film and an excellent property of uniformityof the same compared with the substrate of the single crystal of SrTiP₃.

Then, the property of crystallization on the surface of thesuper-conductive thin film which had been formed in the above-describedmanner was visually observed by utilizing reflective diffraction of anelectron beam at a high speed. The results derived from the visualobservation revealed that a sharp streak-shaped diffraction patternrepresenting a (001) orientation could be obtained with thesuper-conductive thin film formed on the substrate of the single crystalof SrNdGaO₄, and moreover an epitaxially grown film having excellentproperties was formed on the foregoing substrate.

In addition, a super-conductive thin film was formed on the substrate ofthe single crystal of SrNdGaO₄ similar to the aforementioned one underthe same operational conditions as mentioned above while LnBa₂ Cu₃ O₇₋δ(δ=0 or 1, Ln=Yb, Er, Y, Ho, Gd, Eu or Dy) was used as a targetmaterial. It was found from the results derived from visual observationof the super-conductive thin film that a sharp streak-shaped diffractionpattern representing a (001) orientation could be obtained with thesubstrate of the single crystal of SrNdGaO₄, and moreover an epitaxiallygrown film having excellent properties was formed on the substrate ofthe single crystal of SrNdGaO₄.

EMBODIMENT 2

Next, the present invention will be described below with respect to asecond embodiment thereof. According to the second embodiment of thepresent invention, a thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) having athickness of 1000 angstroms was deposited on a substrate of a singlecrystal of SrNdGaO₄ formed with the use of the same method as that inthe first embodiment of the present invention under an atmosphere of amixture of argon and oxygen (having a mixing ratio of 2:1) by employinga RF magnetron spattering method. It should be noted that a materialhaving such a composition that a composition ratio after formation ofthe thin film coincided with that of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) was employedas a target material. In addition, the thin film was formed under theoperational conditions that a gas pressure was set to 5 Pa, an electricpower was set to 200W and a temperature of the substrate was set to 600°C.

After the thin film was deposited in the above-described manner, it wasannealed for one hour at 900° C. under an atmosphere of oxygen.

A device such as a memory device constructed by using super-conductivematerials could be formed while the pattern 12 of the super-conductivethin film formed on the substrate of the single crystal of SrNdGaO₄ inthe above-described manner was employed as a basic structural component.

Thereafter, as shown in FIG. 2, electrodes 31, 32, 33 and 34 each madeof a gold were formed on the super-conductive thin film 12 with the aidof a mask for forming electrodes, and a zero resistance temperatureT_(cO) of the thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) and a criticalsuperconductivity electric current J_(c) of the same at 77° K. weremeasured by employing a four terminal method. In FIG. 2, referencenumeral 14 designates a stabilizing power supply and reference numeral15 designates a potentiometer.

Subsequently, a zero resistance temperature T_(cO) of the thin film ofBi₂ Sr₂ Ca₂ Cu₃ O_(x) formed on the substrate 11 of the single crystalof SrNdGaO₄ and a critical superconductivity electric current J_(c) ofthe same at 77° K. were measured by employing a four terminal method.

The results derived from the measurements are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        material of substrate                                                                           Tc     Jc (A/cm.sup.2)                                      ______________________________________                                        (SrTiO.sub.3)      90    1.5 × 10.sup.4                                 SrNdGaO.sub.4     108      2 × 10.sup.6                                 ______________________________________                                    

As is apparent from the results shown in the table, the case where thesingle crystal of SrNdGaO₄ was used as a substrate is superior inrespect of T_(cO) and J_(c) to the case where the single crystal ofSrTiO₃ was used as a substrate. It is considered that the reason why theformer case is superior to the latter case consists in that thesubstrate of the single crystal of SrNdGaO₄ had an excellent crystalquality of the thin film and an excellent property of uniformity of thesame compared with the substrate of the single crystal of SrTiO₃ andthereby an excellent thin film similar to that in a case of the firstembodiment of the present invention could be obtained when the thin filmof Bi₂ Sr₂ Ca₂ Cu₃ O_(x) was formed on the foregoing substrate.

EMBODIMENT 3

Next, the present invention will be described below with respect to athird embodiment thereof. According to the third embodiment of thepresent invention, a thin film of Tl₂ Ba₂ Ca₂ Cu₃ O_(x) having athickness of 1000 angstroms was deposited on a substrate of a (001)plane single crystal of SrNdGaO₄ which had been formed with the use ofthe same method as that in the first embodiment of the present inventionunder an atmosphere of a mixture of argon/oxygen (having a mixing ratioof 1:1) by employing a RF magnetron spattering method. It should benoted that a material having such a composition that a composition ratioafter formation of the thin film coincided with that of Tl₂ Ba₂ Ca₂ Cu₃O_(x) was employed as a target material, and moreover the operationalconditions for forming the thin film at that time were determined suchthat a gas pressure was set to 10 Pa, an electric power was set to 80 Wand a temperature of the substrate was set to 600° C.

After the thin film was deposited on the substrate in theabove-described manner, an assembly of the substrate and the thin filmwas wrapped in a gold foil and the wrapped assembly was annealed for 10minutes at 905° C. under an atmosphere of oxygen. It should be addedthat the reason why the assemble was wrapped in a gold foil consists inthat vaporization of a thallium should be prevented.

A zero resistance temperature T_(cO) of the thin film of Tl₂ Ba₂ Ca₂ Cu₃O_(x) produced in the above-described manner and a criticalsuperconductivity electric current J_(c) at 77° K. were measured byemploying a four terminal method.

The results derived from the measurements are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        material of substrate                                                                           T.sub.c                                                                              J.sub.c (A/cm.sup.2)                                 ______________________________________                                        SrTiO.sub.3        92    0.5 × 10.sup.4                                 SrNdGaO.sub.4     108      5 × 10.sup.5                                 ______________________________________                                    

Also in this embodiment, as is apparent from the results shown in Table3, the case where the single crystal of SrNdGaO₄ was used as a substrateis superior in respect of T_(cO) and J_(c) to the case where the singlecrystal of SrTiO₃ was used as a substrate. It is considered that thereason why the former case is superior to the latter case consists inthat an excellent thin film of Tl₂ Ba₂ Ca₂ Cu₃ O_(x) could be obtainedin the same manner as in the first embodiment and the second embodimentof the present invention, because the substrate derived from the singlecrystal of SrNdGaO₄ had an excellent crystal quality of the thin filmand an excellent property of uniformity of the same compared with thesubstrate derived from the single crystal of SrTiO₃.

EMBODIMENT 4

Next, the present invention will be described below with respect to amethod of producing a single crystal of Sr_(1-x) Ga_(1-y) Nd_(y)Ga_(1-z) Cr_(z) O₄.

562.58 g of SrCO (having a purity of 99.99%), 20.07 g of CaCO₃ (having apurity of 99.99%), 640.96 g of Nd₂ O₃ (having a purity of 99.99%), 32.67g of La₂ O₃ (having a purity of 99.99%), 345.86 g of Ga₂ O₃ (having apurity of 99.999%) and 24.39 g of Cr₂ O₃ (having a purity of 99.999%)were mixed together and the resultant mixture was calcined at 1000° C.After the calcined mixture was subjected to decarbonizing, the mixturewas crushed and ground and a press-molding operation was performed usingthe pulverized material.

A green compact derived from the press-molding operation was sintered at1300° C. under an environmental atmosphere, whereby a sintered compactof Sr₀.95 Ca₀.05 Nd₀.95 La₀.05 Ga₀.92 Cr₀.08 O₄ having a weight of about1450 g was obtained.

The sintered compact was placed in a crucible of an iridium or platinumhaving an outer diameter of about 80 mm, a height of about 80 mm and awall thickness of 2 mm so that it was molten by high frequency inductionheating. It should be noted that a nitrogen atmosphere containing anoxygen by 0.5 to 2% was employed when the sintered compact was molten inthe crucible made of an iridium. In contrast with this, when thesintered compact was molten in the crucible made of a platinum, anitrogen atmosphere containing an oxygen by 10 to 21% was used. Thereason why an oxygen was added to the atmosphere consists indecomposition and vaporization of a gallium oxide should be prevented.

After the sintered compact of Sr₀.95 Ca₀.05 Nd₀.95 La₀.05 Ga₀.92 Cr₀.08O₄ was molten in the above-described manner, a single crystal having theforegoing composition was grown by employing a Czochralski method forpulling up a molten substrate.

A (100) axis single crystal of SrNdGaO₄ was used as a seed crystal. As aresult, a 100) axis single crystal of SrNdGaO₄ having a diameter of 30mm and a length of 70 mm could be obtained under the operationalconditions that a pulling-up speed was set to 1 mm/hr and a rotationalspeed of the single crystal was set to 25 rpm.

The thus obtained single crystal of SrNdGaO₄ containing Ca, La and Cr asimpurities was sliced so as to successively produce wafer-shapedsubstrates each composed of a (001) plane single crystal. Then, a thinfilm of YBa₂ Cu₃ O₇₋δ having a thickness of 1000 angstroms was depositedon the wafer-shaped substrate of a SrNdGaO₄ based (001) plane singlecrystal under an atmosphere of a mixture of argon and oxygen (having amixing ratio of 1:1) by employing a RF magnetron spattering method. Acomposition ratio of a target material was adjusted such that thecomposition ratio of the thin film after formation of the same coincidedwith that of YBa2₂ Cu₃ O₇₋δ. The operational conditions for growing thesingle crystal were determined such that a gas pressure was set to 10Pa, an electric power was set to 300W and a temperature of the substratewas set to 600° C.

After the thin film was deposited on the substrate, an assembly of thesubstrate and the thin film was annealed for one hour at 900° C. underan oxygen atmosphere.

Thereafter, electrodes each composed of a gold were formed on theassembly with the aid of a mask for forming the electrodes by employinga vacuum vaporizing/depositing method.

A zero resistance temperature T_(cO) of the thus formed thin film ofYBa₂ Cu₃ O₇₋δ and a critical superconductivity electric current J_(c) ofthe same at 77° K. were measured by employing a four terminal method.

The results derived from the measurements are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    mol ratio                                                                     Sr   Ca   Nd   La   Ga   Cr   T.sub.c0                                                                         J.sub.c (A/cm.sup.2)                         __________________________________________________________________________    1    0    1    0    1    0    85 1 × 10.sup.5                           0.95 0.05 0.95 0.05 0.92 0.08 87 5 × 10.sup.5                           0.9994                                                                             0.0006                                                                             1    0    1    0    88 3 × 10.sup.5                           1    0    0.9994                                                                             0.0006                                                                             1    0    87 3 × 10.sup.5                           1    0    1    0    0.9994                                                                             0.0006                                                                             87 2.5 × 10.sup.5                         __________________________________________________________________________

For the purposes of comparison, a thin film of YBa₂ Cu₃ O₇₋.tbd. wasdeposited on the substrate of the single crystal of SrNdGaO₄ containingCa, La and Cr as impurities by a different content under the sameoperational conditions as those mentioned above. The results derivedfrom the measurements conducted for determining the zero resistancetemperature T_(cO) and the critical superconductivity electric currentJ_(c) at 77° K. are as shown in Table 1.

As is apparent from the results shown in Table 4, properties of thesingle crystal represented by the zero resistance temperature T_(cO) andthe critical superconductivity electric current J_(c) at 77° K. weresubstantially improved by adding Ca, La, and Cr to the substrate asimpurities.

When a content of each of these elements represented by a mol ratio waslower than 0.00001, no significant effect was recognized. On thecontrary, when the content of the same is higher than 0.1, cracks orcells readily appeared on the single crystal.

It should be noted that only one of the aforementioned impurities may becontained in the substrate or two or three kinds of impurities may becontained in the substrate in the combined state.

EMBODIMENT 5

Next, the present invention will be described below with respect to afifth embodiment of the present invention. According to the fifthembodiment of the present invention, a single crystal of SrNdGaO₄containing Ca, La and Cr as impurities which was produced with the useof the same method as that in the fourth embodiment of the presentinvention was sliced so as to successively form wafer-shaped substrateseach constituted by a (001) plane single crystal. Then, a thin film ofBi₂ Sr₂ Ca₂ Cu₃ O_(x) having a thickness of 1000 angstroms was depositedon the wafer-shaped substrate derived from the single crystal ofSrNdGaO₄ under an atmosphere of a mixture of argon and oxygen (having amixing ratio of 2:1) by employing a RF magnetron spattering method. Inthis embodiment, the composition ratio of a target material was adjustedsuch that the composition ratio after formation of the thin filmcoincided with that of Bi₂ Sr₂ Ca₂ Cu₃ O_(x). It should be added thatformation of the thin film was accomplished under the operationalconditions that a gas pressure was set to 5 Pa, an electric power wasset to 200 W and a temperature of the substrate was set to 600° C.

After the thin film was deposited on the substrate, an assembly of thesubstrate and the thin film was annealed for one hour at 900° C. underan oxygen atmosphere.

Electrodes each composed of gold were formed on the thus produced thinfilm of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) in the same manner as in the fourthembodiment of the present invention, and a zero resistance electriccurrent T_(cO) and a critical superconductivity electric current j_(c)77° K. were measured by employing a four terminal method.

The results derived from the measurement are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    mol ratio                                                                     Sr   Ca   Nd   La   Ga   Cr   T.sub.c0                                                                         J.sub.c (A/cm.sup.2)                         __________________________________________________________________________    1    0    1    0    1    0    108                                                                              2 × 10.sup.6                           0.95 0.05 0.95 0.05 0.92 0.08 110                                                                              5 × 10.sup.6                           0.9994                                                                             0.0006                                                                             1    0    1    0    111                                                                              4 × 10.sup.6                           1    0    0.9994                                                                             0.0006                                                                             1    0    110                                                                              4 × 10.sup.6                           1    0    1    0    0.9994                                                                             0.0006                                                                             110                                                                              4 × 10.sup.6                           __________________________________________________________________________

As is apparent from the results shown in Table 5, properties of thesingle crystal represented by the zero resistance temperature T_(cO) andthe critical superconductivity electric current J_(c) at 77° K. weresubstantially improved by adding Ca, La and Cr to the substrate asimpurities.

It is considered that when the thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) wasformed in the above-described manner, a thin film having the sameexcellent properties as those in the first embodiment of the presentinvention could be obtained.

EMBODIMENT 6

Next, the present invention will be described below with respect to asixth embodiment of the present invention. According to the sixthembodiment of the present invention, a single crystal of SrNdGaO₄containing Ca, La and Cr as impurities which was produced by employingthe same method as that in the fourth embodiment of the presentinvention was sliced to successively form wafer-shaped substrates eachconstituted by a (001) plane single crystal. Then, a thin film of Tl₂Ba₂ Ca₂ Cu₃ O_(x) having a thickness of 1000 angstroms was deposited onthe wafer-shaped substrate derived from the single crystal of SrNdGaO₄under an atmosphere of a mixture of argon and oxygen (having a mixingratio of 1:1) by employing a RF magnetron spattering method. In thisembodiment, the composition ratio of a target material was adjusted suchthat the composition ratio after formation of the thin film coincidedwith that of Tl₂ Ba₂ Ca₂ Cu₃ O_(x). It should be added that formation ofthe thin film was accomplished under the operational conditions that agas pressure was set to 10Pa, an electric power was set to 80 W and atemperature of the substrate was set to 600° C.

After the thin film was deposited on the substrate, an assembly of thesubstrate and the thin film was wrapped in a gold foil and the wrappedassembly was annealed for 10 minutes at 905° C. under an oxygenatmosphere. It should be noted that the reason why the assembly waswrapped in a gold foil consists in that vaporization of a thalliumshould be prevented.

Electrodes each composed of gold were formed on the thus produced thinfilm of Tl₂ Ba₂ Ca₂ Cu₃ O_(x) in the same manner as in the fourthembodiment and the fifth embodiment of the present invention, and a zeroresistance temperature T_(cO) and a critical superconductivity electriccurrent J_(c) at 77° K. were measured by employing a four terminalmethod.

The results derived from the measurements are shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________    mol ratio                                                                     Sr   Ca   Nd   La   Ga   Cr   T.sub.c0                                                                         J.sub.c (A/cm.sup.2)                         __________________________________________________________________________    1    0    1    0    1    0    108                                                                                5 × 10.sup.5                         0.95 0.05 0.95 0.05 0.92 0.08 112                                                                              2.1 × 10.sup.6                         0.9994                                                                             0.0006                                                                             1    0    1    0    111                                                                              1.8 × 10.sup.6                         1    0    0.9994                                                                             0.0006                                                                             1    0    112                                                                              1.9 × 10.sup.6                         1    0    1    0    0.9994                                                                             0.0006                                                                             111                                                                              1.8 × 10.sup.6                         __________________________________________________________________________

Also in this embodiment, as is apparent from the results shown in Table6, properties of the assembly represented by the zero resistancetemperature T_(cO) and the critical superconductivity electric currentJ_(c) at 77° K. were substantially improved by adding Ca, La and Cr tothe substrate as impurities.

It is considered that also in the case where the thin film of Tl₂ Ba₂Ca₂ Cu₃ O_(x) was formed in the above-described manner, a thin filmhaving the same excellent properties as those in the fourth embodimentand the fifth embodiment of the present invention could be obtained.

It should of course be understood that the present invention should notbe limited only to the aforementioned several embodiments thereof butthe present invention may equally be applied to a case where asuper-conductive thin film composed of other oxide is formed.

According to each of the aforementioned embodiments of the presentinvention, a RF magnetron spattering method was employed to form asuper-conductive thin film composed of an oxide. However, the presentinvention should not be limited only to this method. Alternatively, avacuum vaporizing/ depositing method, a hypercomplexvaporizing/depositing method, a molecular beam epitaxy method, a MSDmethod or the like may be employed for carrying out the presentinvention.

Further, each of the aforementioned embodiments of the present inventionhas been described above with respect to a case where a (001) plane wasemployed as a reference orientation plane. However, the sameadvantageous effects as those mentioned above can be obtained also in acase where a (100) plane and a (110) plane are employed.

In addition, according to each of the aforementioned embodiments of thepresent invention, Ca, La and Cr were added to a substrate asimpurities. Alternatively, Ba, Mg, Na, K, Al, Fe, Si, Ce, Pr and Zr maybe added to the substrate with some advantageous effect but without anyclearly recognizable advantageous effects as obtained in the case whereCa, La and Cr were added as impurities.

INDUSTRIAL APPLICABILITY

As will readily be apparent from the above description, the presentinvention has made it possible to practically use various kinds ofsuper-conductive devices.

I claim:
 1. A substrate of a single crystal, said crystal comprising atleast one element selected from a group consisting of calcium having amol ratio relative to strontium within the range of 0.00001 to 0.1 mol,lanthanum having a mol ratio relative to neodymium within the range of0.00001 to 0.1 mol, chromium having a mol ratio relative to galliumwithin the range of 0.00001 to 0.1 mol and mixtures thereof, which saidelements are added to a initial said compound having the formulaSrNdGaO₄, said compound having a K₂ NiF₄ type crystal structure.